1. Field of the Invention
This invention relates to an improved method of and apparatus for measuring the injection rate of an injection valve, and more particularly to the injection rate of a fuel injection valve.
2. Description of the Prior Art
In the production and function testing of fuel injection components, such as injection valves, common rail injectors, and other high-pressure injection valves, various testing devices and methods for measuring quantity are described in the prior art. For instance, from German Patent Disclosure DE 100 64 511 A1, the measurement piston principle is known, in which the injection valve injects fuel into a measurement volume filled with a test medium. The pressure in the measurement volume is kept constant by providing that a measurement piston is positively displaced by the injection quantity. From the displacement of the measurement piston, the injection quantity can then be calculated directly. Because of the mechanical piston motion, this method is dynamically limited, and as a result it cannot meet the increasingly stringent demands for chronologically high-resolution measurement of the injection rate in modern high-pressure injection systems for internal combustion engines, which often include a plurality of partial injections per injection cycle.
An alternative and precise method, as described for instance in W. Zeuch, “Neue Verfahren zur Messung des Einspritzgesetzes und der Einspritz-Regelmäβigkeit von Diesel-Einspritzpumpen”, Motortechnische Zeitschrift (MTZ) [“New Methods for Measuring the Injection Principle and the Regularity of Injection of Diesel Injection Pumps”, Automotive Engineering Journal] 22 (1961), pp. 344–349, is the hydraulic pressure increase method (HDV). In it, the injection valve likewise injects into a liquid-filled measurement volume, but here it is the measurement volume that is kept constant. As a result, a pressure increase occurs in the measurement volume and is measured by a suitable pressure sensor. Modern piezoelectrically-based pressure sensors are distinguished by a very fast response time, which makes chronologically high-resolution measurements possible. From the course over time of the pressure increase, both the course of the injection rate and the injection quantity can in principle be calculated.
In practice, however, this is made more difficult by a number of factors: In the measurement volume V, the injected fuel causes pressure oscillations in the corresponding natural frequencies of the measurement volume, and these natural frequencies depend on the geometric dimensions of the measurement volume. Besides the fundamental oscillation, as a rule many harmonics are also induced, and as a rule a plurality of oscillation modes are possible. This makes filtering of the pressure sensor measurement signal more difficult, since the frequencies of the natural oscillations are partly in the range of the frequencies of the measurement signal.
Precise measurement of the absolute value of the injection quantity Δm is also made more difficult by the fact that the measured magnitude of the pressure must first be converted to the injected liquid quantity. The conversion factors include the modulus of compression and the density. These variables depend on the test conditions and the prior history in question and are therefore not available from earlier measurements with the requisite precision. To ascertain these variables, a separate, complicated calibration operation is necessary for each measurement, which makes the measurement inconvenient and in practice hard to perform. To that end, via a separate calibration cylinder, a defined calibration volume ΔVk is introduced into the measurement volume V, and the pressure change Δpk is measured. The modulus of compression K is then obtained from the equationK=Δpk/ΔVk·V  (I)
The injected volume ΔV can thus be calculated as follows:ΔV=V/K·Δp 
In order finally to calculate the injection quantity, a conversion to mass is necessary, which requires knowledge of the density ρ:Δm=ρ·ΔV=V·ρ/K·Δp 
The density depends on the temperature of the test medium. To take this into account, the temperature is measured by means of a temperature sensor in the measurement volume, and the density is corrected accordingly. The temperature measurement is pointwise and does not take any possibly unequal temperature in the entire measurement volume into account.
For ascertaining the modulus of compression K by the above equation (I), it is necessary to introduce a defined calibration volume into the measurement volume, which makes a separate volume transducer necessary. Furthermore, there is the disadvantage that for the calibration measurement, a separate measurement time is necessary, which lessens the possible frequency of successive measurements.